The present invention relates to hermetically sealed cases or housings suitable for implantation within living tissue, and more particularly, to a method and apparatus for hermetically sealing an implantable device which prevents moisture from forming within the sealed device. In a preferred approach, the method and apparatus utilize a tubular feedthrough to vent moisture and other volatile gases trapped within the sealed device during the manufacturing process. Such tubular feedthrough also facilitates hermeticity testing of the implantable device during the manufacturing process.
Hermetically sealed cases or housings are widely used to protect electronic or other components that may be susceptible to damage or malfunction from exposure to the surrounding environment. The hermetic seal is simply an airtight, durable seal that is long-lasting and physically rugged. Sometimes the interior of an hermetically sealed enclosure is filled with an inert gas such as helium, to further retard the deterioration of the component or components inside. As no seal is perfect, the tightness of the hermetic seal, referred to as the hermeticity, is typically measured or specified in terms of the leakage rate through the seal, expressed in cc/sec of helium at standard temperature and pressure. Sometimes, for very low leakage rates, the hermeticity can only be measured by placing a radioactive gas within the enclosure and then using an appropriate radiation detector to "sniff" the seal for radioactive leaks.
Where the electrical component or components are to be implanted in body tissue, the hermetically sealed case (which must be made from a material that is compatible with body tissue, such as titanium, platinum or stainless steel or glass) serves a dual purpose: (1) it protects the electrical or other components housed in the device from body fluids and tissue, which fluids and tissue could otherwise prevent the components from performing their desired function; and (2) it protects the body tissue and fluids from the electrical or other components, which components may be made at least in part from materials that may be damaging to body tissue, and which therefore could pose a potential health risk to the patient wherein they are implanted. It is thus critically important that the hermetic seal of an implanted device be especially long-lasting and physically rugged. For this reason, stringent requirements are imposed on the hermeticity of an implanted device, typically requiring a seal that provides a helium leakage rate of less than 10.sup.-8 cc/sec at standard temperature and pressure (STP).
In recent years, the size of implanted medical devices has decreased dramatically. It is now possible, for example, to construct a simple stimulator device in a small hermetically sealed glass tube that can be implanted through the lumen of a needle. With such a small size comes increased requirements for the tightness of the hermetic seal because there is less empty space inside of the sealed unit to hold the moisture that eventually leaks therethrough. The hermeticity requirements of such small devices may thus be on the order of 10.sup.-11 or 10.sup.-12 cc/sec. While the small size is thus advantageous, the stringent hermeticity requirements imposed for such small devices makes them extremely difficult to manufacture, and thus increases the cost.
Most implanted medical devices, such as a cardiac pacemaker, neural stimulator, biochemical sensor, and the like, require hermetic conductive feedthroughs in order to establish electrical contact between the appropriate circuitry sealed in the hermetically closed case or capsule and an external electrode that must be in contact with the body tissue or fluids outside of the sealed case or capsule. In a pacemaker, for example, it is common to provide such a feedthrough by using a feedthrough capacitor. A representative feedthrough capacitor is described in U.S. Pat. No. 4,152,540. Alternatively, a hermetic feedthrough is typically used to establish electrical connections between the appropriate electronic components or circuitry sealed in the hermetically closed case or capsule and an external control device, or monitoring equipment.
Heretofore, an hermetic feedthrough for implantable devices has consisted of a ceramic or glass bead that is bonded chemically at its perimeter through brazing or the use of oxides, and/or mechanically bonded through compression, to the walls of the sealed case or capsule. A suitable wire or other conductor passes through the center of the bead, which wire or conductor must also be sealed to the bead through chemical bonds and/or mechanical compression. The feedthrough is thus circular, and the wire(s) or conductor(s) mounted within the bead are centered or mounted in a uniform pattern centrally positioned within the bead. Such centering is necessary due to the thermal coefficients required for the different expansion rates that occur when heat is applied to either create a compression seal or to create an oxide or bronze bond.
A significant problem associated with these hermetically sealed devices, particularly where the device is implanted in living tissue, is the inability to effectively seal the device with conventional feedthrough mechanisms. During existing sealing process, glass beads are fused within the device cases or capsules to hermetically seal the case or capsule. Water vapors are often produced by the gas flame fusing and are often trapped inside the capsules which water vapors may lead to eventual failure of the implanted device.
Another problem associate with conventional sealing processes is related to the expansion of gases remaining inside the sealed capsule. As the glass bead is fused to the case or capsule, air inside the case or capsule expands. The expanding air tries to escape from the case or capsule and is likely to result in localized stress in the glass fusion areas and may even form a hole through the molten glass. If a hole forms within the glass, it is very difficult if not impossible to repair.
Alternative methods of sealing microstimulator capsules involve expensive equipment such as an infrared laser or a dedicated glass diode sealing machine which utilizes heated formed graphic holding blocks.
Other examples of the related art pertaining to hermetic feedthrough assemblies for implantable medical devices are disclosed in U.S. Pat. No. 4,678,868 issued to Kraska et al., and U.S. Pat. No. 4,940,858 issued to Taylor et al. While these patents are directed to feedthrough assemblies for implanted medical devices, they do not address the problem of trapped water vapors and expanding air within the sealed device.
Still other art relating generally to methods for forming hermetically sealed cases having electrical feedthroughs and vias include U.S. Pat. No. 4,525,766 issued to Petersen, U.S. Pat. No. 4,861,641 issued to Foster et al., and U.S. Pat. No. 4,882,298 issued to Moeller et al. While these patents teach improvements in the art, such teachings are limited to use with semiconductor substrates and are not easily adaptable for use with microminiature devices implantable within living tissue.
It is thus evident that what is needed is a cost-effective manner of encapsulating implantable electronic or other devices that eliminates or prevents any moisture and expanding air from becoming trapped within the sealed device as the device is hermetically sealed. Further, once such device is sealed, there is a need for a cost-effective, non-destructive manner of testing the hermeticity of the sealed device during the manufacturing process.